This invention relates to a method of manufacturing a hermetic sealing member having improved airtightness and which can be manufactured at a low cost.
Generally, in an electronic tube, for the purpose of improving electrical insulating strength between electrodes and providing, strength and durability, a sealing structure is widely used in which a ceramic member and a metal member are bonded together with solder.
FIG. 1 shows one example of a magnetron tube utilizing a seal member in which a ceramic member and a metal member are soldered together. The magnetron tube comprises an anode cylinder 1 and vanes 2 defining a plurality of resonance cavities divided in radial direction, the vanes 2 and the anode cylinders constituting an anode electrode 3. At the center of the anode electrode 3 is disposed a cathode electrode 4 made of a helically wound electric heating wire 4 with a predetermined spacing between it and the vanes to form an interaction space 5. Circular end plates 6a and 6b are secured to the opposite ends of the anode electrode 1 by soldering or arc welding. Permanent magnets 7a and 7b are secured to the inner surfaces of the end plates by magnet holders 8a and 8b to form a magnetic circuit A and to concentrate magnetic flux in the interaction space 5. A metal cylinder 9 made of Kovar, for example, is soldered to the outer surface of the end plate 6a and a ceramic cylinder 10 of an alumina ceramic is soldered to the other end of the metal cylinder 9 to form a hermetic seal structure. An exhaust pipe 11 formed by chipping off a copper tube is secured to the other end of the ceramic cylinder to form an enclosure. The exhaust pipe 11 is airtightly connected to an antenna lead wire 12 connected to a portion of a vane 2. A metal cover 13 is secured to the outer periphery of the exhaust pipe 11 for protecting the airtight connection and for preventing an electric arc from being caused by excessive concentration of electric field.
The metal cover 13 is also used as a high frequency antenna. A metal cylinder 14 made of Kovar, for example, is soldered to the outer surface of the end plate 6b, and a stem 15 for supporting the cathode electrode 4 is soldered to the opposite end of the metal cylinder 14.
In the magnetron constructed as described above, when a predetermined filament voltage is applied to the outer end of the stem, the cathode electrode 4 will be heated and when a predetermined voltage is applied to the anode electrode 3, the cathode electrode 4 will emit thermoelectrons. When these electrons whirl in the magnetic field in the interaction space 5, high frequency oscillations are produced in the interaction space and the oscillation output is radiated into an oven of a high frequency range, for example, through the antenna conductor 12 and through the metal cover 13 acting as a high frequency antenna to heat or defrost foodstuffs.
FIG. 2 is a longitudinal sectional view showing the details of the soldered sealing structure shown in FIG. 1. The ceramic cylinder 10 consists essentially of alumina with its end surface metallized by sintering a powder of molybdenum and the cylinder 9 is made of Kovar and has a wall thickness of from 0.3 to 1 mm. The ceramic cylinder 10 and the metal cylinder 9 are hermetically bonded together by interposing a solder 16 such as a silver-copper alloy between the ends thereof and then heating the assembly in a reducing or inert atmosphere.
To obtain a high quality hermetic seal structure, it is necessary to carefully select the soldering temperature, the purity of the solder, the pressure applied to the parts at the time of soldering. To satisfactorily solder a metal cylinder and a ceramic cylinder, it is necessary to finish the end surfaces to be soldered with perfect flatness. Usually, the irregularities of these end surfaces should be limited to be less than 0.1 mm. Otherwise small gaps may be formed and thus perfect airtightness could not be assured or the strength of the hermetic bond would be decreased. For this reason, one end of the metal cylinder 9 is cut away along line C--C shown in FIG. 3, which increases the cost of manufacturing.
Although such metal cylinders 9 may be cut out from a length of a metal pipe, the metal pipe is relatively expensive and repeated cutting is not efficient. For this reason, it has been the practice to prepare a metal cylinder 9 by pressing according to the steps a, b and c shown in FIG. 4.
More particularly, a Kovar plate 17 shown in FIG. 4a is deformed into a cup-shaped member 18 as shown in FIG. 4b and then the flange and the bottom of the cup-shaped member 18 are removed to obtain a cylindrical body as shown in FIG. 4c. With this method, however, when removing the flange and the bottom, it is difficult to make uniform the circumferential clearance between the cylindrical body and the metal mold or die, not shown. Moreover, at the time of deforming, the radius of curvature at the inner periphery of the flange is not always uniform so that the flatness of the resulting cylindrical body is impaired.
To obviate these difficulties, the opposite ends of the cylindrical body were sheared perpendicularly to the axis thereof. But, difficulties are encountered in fabricating a metal mold which can serve for the cylinder squeezing and the shearing in combination and such a metal mold becomes very expensive. Separate metal molds for these purposes are also expensive.